(1) Field of the Invention
The present invention relates to the removal of carbon dioxide and hydrogen sulfide from gaseous streams utilizing a CO.sub.2 selective absorbent in admixture with an aqueous alkaline solution containing a chelated polyvalent metal.
(2) Description of the Prior Art
In U.S. Pat. No. 4,091,073, the use of an absorbent for carbon dioxide and preferably for hydrogen sulfide as well, is taught in a process for removal of hydrogen sulfide from gaseous streams by contact with a polyvalent metal chelate. The use of carbon dioxide absorbents in a process for the removal of hydrogen sulfide from a gaseous stream is also taught in U.S. Pat. No. 4,518,576 and U.S. Pat. No. 4,368,178.
The removal of hydrogen sulfide and carbon dioxide from sour gaseous streams is disclosed in U.S. Pat. No. 4,402,930 by scrubbing the gaseous streams with an aqueous solution containing a CO.sub.2 absorbent and a polyvalent metal chelate in a higher valence state and oxidizing said chelate in a regeneration zone. It is known in the prior art that iron in the ferric state acts as a catalyst for the oxidation of ethylenediamine tetraacetic acid in aqueous solutions from Motekaitis, et al. Canadian Journal Of Chemistry, volume 58, No. 19, Oct. 1, 1980.
One of the main disadvantages of the processes for removing hydrogen sulfide from gaseous streams utilizing polyvalent metal chelates is the instability of the chelating agent under the process conditions. In order to overcome the instability of the chelating agents, particularly those complexed with polyvalent metal ions such as iron, the prior art has taught the use of mixtures of certain chelating agents. In U.S. Pat. No. 4,421,733 and U.S. Pat. No. 4,455,287, methods are disclosed for reducing the instability of polyvalent metal chelating agents under the reaction conditions in which these agents are utilized to remove hydrogen sulfide from gaseous streams. In U.S. Pat. No. 4,421,733, the use of a stabilizing amount of bisulfite ion is suggested and in U.S. Pat. No. 4,455,287, the use of a biocide is suggested as means of stabilizing a polyvalent metal chelate for use in the removal of hydrogen sulfide gas from a fluid stream. In U.S. Pat. No. 3,068,065 there is disclosed a process for the removal of hydrogen sulfide from gaseous streams by washing the gas stream with a solution containing an iron chelate wherein the iron is present in the chelate in the ferric state.
British Pat. No. 990,800 issued July 28, 1965 to Humphreys and Glasgow Ltd. teaches the benefit of employing a high proportion of a polyvalent metal chelate in the reduced valence state in conjunction with a polyvalent metal chelate in the oxidized or higher valence state, to reduce degradation of the chelating agent in a process for the removal of hydrogen sulfide from a gas. The gaseous stream is contacted with an aqueous solution containing iron complexed with an amino polycarboxylic acid in which the iron is a mixture of the higher and lower valence state. The hydrogen sulfide gas is converted to sulfur by contact with the iron chelating agent in which the iron is present in the higher oxidation state. In turn, the iron is reduced to the lower oxidation state. Subsequently, the iron is converted from the lower oxidation state to the higher oxidation state in an oxidation zone and it is at this point, that, as the iron chelating agent is exposed to oxidation, there results a progressive loss of the chelating agent from the aqueous solution. Precipitation of insoluble iron compounds occurs as the result of the decomposition of the iron chelate. The British Patent teaches the controlled, oxidative regeneration of the iron chelate so as to prevent localized, intensive, oxidative decomposition of the chelating agent. Generally from 15 to 75% by weight of the total iron present in the iron chelate solution can be ferrous iron with the preferred proportion of ferrous iron chelate remaining in the solution after regeneration being between 20 and 50% by weight, based upon the total iron chelate present in said solution.
There is no suggestion in any of these prior art references for the use of a polyvalent metal chelate in a contact zone, Particularly an iron chelate wherein all the iron is present in the chelate as the reduced state of the metal. In addition, there is no suggestion for the use of mixed higher and lower valence state polyvalent metal chelates wherein the amount in moles of said chelate present in the lower valence state is greater than about 5 times the amount present of the higher valence polyvalent metal chelate. After hydrogen sulfide is absorbed in the process of the invention in a contact zone by contacting a gaseous stream with an aqueous alkaline solution and converted to hydrosulfide and/or sulfide ions, some or all of these ions may be converted in the contact zone to elemental sulfur by reaction with any iron chelate which may be present in the higher valence state. The remainder of these ions are converted in an oxidation zone to elemental sulfur. The conversion is carried out in the oxidation zone by contact with an iron chelate present in a higher valence state only in at least as effective amount.